The present invention relates to a new and improved construction of an optical smoke detector which is of the type comprising a radiation source which produces a directed bundle or beam of radiation, a scattered radiation-receiver arrangement having directional characteristic, and an evaluation circuit connected with the radiation receiver arrangement for delivering signals when the radiation scattering at the smoke particles in the radiation beam exceeds a predetermined value or degree.
With such type smoke detectors it is possible to detect or measure electromagnetic radiation which is scattered at small particles suspended in a medium. As the radiation there can be beneficially employed, for instance, light, infrared or ultraviolet radiation. As the medium there can be used different types of transparent gases, for instance air, or also liquids.
A particularly useful field of application of such detectors is as fire alarms, wherein the smoke particles suspended in air can be detected and by means of an evaluation circuit there can be triggered an alarm signal whenever the intensity of the smoke, and thus the intensity of the scattered radiation, exceeds a predetermined value. With such scattered radiation or smoke detectors, extreme care must be taken to ensure that the radiation receiver is not disturbed by other radiation. In order to prevent, for instance, radiation emanating from the surroundings from impinging upon the receiver, such equipment is therefore equipped with a light impervious housing surrounding the bundle or beam of radiation. Internally of the housing measures must be provided for extensively absorbing the radiation at those locations where the radiation beam impinges upon the inner wall of the housing, for instance with the aid of suitable radiation traps. Notwithstanding these measures, it is impossible to prevent that, for instance, due to reflection or bending of the incident radiation, the radiation receiver nonetheless receives a certain amount of spurious or unwanted radiation. With such detectors of the state-of-the-art it therefore was not possible to record scattered radiation of low intensity, and the sensitivity could not be increased to an extent as would be necessary for many fields of application.
Now in order to eliminate such drawback it has already been proposed to provide prior art smoke detectors with two radiation receivers, the fields of view of which detect different parts of the radiation beam, which are located at different distances from the radiation source, and in the presence of smoke, at which the scattered radiation intensity differs. By providing logical circuitry an alarm signal only then is triggered when the received intensities of the scattered radiation are in a proper correlation or relationship to one another. Since the fields of view of both receivers however encompass different wall regions of the detector housing it is not possible to eliminate different reflection characteristics at these locations which, for instance, can be caused by dissimilar dust deposits.
According to another prior art smoke detector there is likewise provided two radiation receivers arranged within a measuring housing and at which impinge different proportions of scattered radiation caused by penetration of smoke into the measuring housing. One of the radiation receivers faces the radiation beam, whereas the other radiation receiver faces away from the radiation beam so that it only receives the scattered radiation which is reflected at the housing wall. Also in this case both of the scattered radiation receivers are directed towards different parts of the housing, i.e., the one scattered radiation receiver detects reflections occurring at the rear of the other receiver. This construction equally does not ensure that both radiation receivers will truly receive the same amount of spurious radiation in the presence of scattering particles or smoke. Also with such detector it is possible for the presence of smoke to be falsified due to different reflection characteristics of the walls or housing parts which are located in the fields of view of both scattered radiation receivers.